Without limiting the scope of the invention, its background is described in connection with compositions and methods for controlling the duration of repolarization of the cardiac ventricle QT in a subject.
The beating of the heart is due to precisely controlled regularly spaced waves of myocardial excitation and contraction. The electrical currents during ion-based depolarization and repolarization can be measured by electrical leads placed on the body in specific locations (the electrocardiogram) which measure electrical waves. The P-wave represents a wave of depolarization in the atrium. When the entire atria becomes depolarized, the wave returns to zero. After 0.1 seconds the ventricle is entirely depolarized resulting in the QRS complex. The three peaks are due to the way the current spreads in the ventricles. This is followed by the T-wave or repolarization of the ventricle. The QT interval measured from the beginning of the QRS complex to the end of the T wave on the standard ECG represents the duration till the completion of the repolarization phase of the cardiac myocyte (or the depolarization and repolarization of the ventricle). The duration of this interval can vary due to genetic variation, cardiac disease, electrolyte balance, envenomation, and drug treatments. Prolongation of the QT interval can result in ventricular arrhythmias and sudden death.
Drug induced long QTc Syndrome (LQTS) i.e., a prolongation of the action potential duration is a common cause of governmental mandated drug withdrawal. QTc prolongation is an unpredictable risk factor for Torsades de Pointes (TdP), a polymorphic ventricular tachycardia leading to ventricular fibrillation. Drug induced LQTS comprises about 3% of all prescriptions which when followed by TdP may constitute a lethal adverse reaction. Patients taking one or more than one QTc-prolonging drug concomitantly, have an enhanced risk of TdP. While the overall occurrence of TdP is statistically rare, it is clinically significant for the affected individual. Testing for this drug effect is a mandatory requirement prior to allowing a drug to enter clinical trials.
Common structurally diverse drugs block the human ether-a-go-go-related gene (KCNH2 or hERG) coded K+ channel and the cardiac delayed-rectifier potassium current IK (KV11.1) resulting in acquired LQTS. Drug-associated increased risk of LQTS is a major drug development hurdle and many drugs have been withdrawn during pre-clinical development, assigned black box warnings following approval or withdrawn from the market. Autosomal recessive or dominant LQTS based upon 500 possible mutations in 10 different genes coding for the potassium channel has an incidence of 1:3000. Prolonged QT intervals, or risk of LQTS occur in 2.5% of the asymptomatic US population. This syndrome when expressed can lead to severe cardiac arrhythmia and sudden death in untreated patients. The probability of cardiac death in patients with asymptomatic congenital LQTS who are medicated with LQTS-inducing drugs is increased.
The majority of the acquired LTQS drug withdrawals are due to obstruction of the potassium ion channels coded by the human ether-a-go-go related gene (hERG). High concentrations of hERG blocking drugs generally induce a prolonged QTc interval and increase the probability of TdP. Up to 10% of cases of drug-induced TdP can be due to 13 major genetic mutations, 471 different mutations, and 124 polymorphisms (Chig, C 2006).
Systems and methods for detection of LQTS have been described previously. For example U.S. Patent Publication No. 2010/0004549 (Kohls et al. 2010) discloses a system and method of detecting LQTS in a patient by comparing a collected set of ECG data from the patient to a plurality of databases of collected ECG data. The plurality of databases will include a database containing previous ECGs from the patient, a known acquired LQTS characteristics database, and a known genetic LQTS characteristics database. Comparing the patient's ECG to these databases will facilitate the detection of such occurrences as changes in QT interval from success of ECGs, changes in T-wave morphology, changes in U-wave morphology, and can match known genetic patterns of LQTS. The system and method is sensitive to patient gender and ethnicity, as these factors have been shown to effect LQTS, and is furthermore capable of matching a QT duration to a database of drug effects. The system and method is also easily integrated into current ECG management systems and storage devices.
A system and method for the diagnosis and treatment of LQTS is described in U.S. Patent Publication No. 2008/0255464 (Michael, 2008). The Michael invention includes a system for diagnosing LQTS, which derives a QT/QS2 ratio from an electrical systole (QT) and a mechanical systole (QS2) to detect a prolonged QT interval in a patient's cardiac cycle. A processor acquires the systoles from a microphone and chest electrodes, calculates the QT/QS2 ratio, and outputs the result to a display. The processor may compare the QT/QS2 ratio to a threshold value stored in memory for diagnosing LQTS in the patient. A user interface provides for programming, set-up, and customizing the display. A mode selector allows the system to operate alternatively as a phonocardiograph, a 12 lead electrocardiograph, or a machine for diagnosing LQTS. A related method for diagnosing cardiac disorders such as LQTS includes measuring QT and QS2 during a same cardiac cycle, calculating a QT/QS2 ratio, and comparing the result to a threshold value derived from empirical data. The method may include measuring systoles both at rest and during exercise, and may be used for drug efficacy, dosage optimization, and acquired LQTS causality tests.
A method for the treatment of cardiac arrhythmias is provided in U.S. Patent Publication No. 2007/0048284 (Donahue and Marban, 2007). The method includes administering an amount of at least one polynucleotide that modulates an electrical property of the heart. The polynucleotides of the invention may also be used with a microdelivery vehicle such as cationic liposomes and adenoviral vectors.
Methods, compositions, dosing regimes, and routes of administration for the treatment or prevention of arrhythmias have been described by Fedida et al. (2010) in U.S. Patent Publication No. 2001/00120890. In the Fedida invention, early after depolarizations and prolongation of QT interval may be reduced or eliminated by administering ion channel modulating compounds to a subject in need thereof. The ion channel modulating compounds may be cycloalkylamine ether compounds, particularly cyclohexylamine ether compounds. Also described are compositions of ion channel modulating compounds and drugs which induce early after depolarizations, prolongation of QT interval and/or Torsades de Pointes. The Fedida invention also discloses antioxidants which may be provided in combination with the ion channel modulating compounds, non-limiting examples of the antioxidants include vitamin C, vitamin E, beta-carotene, lutein, lycopene, vitamin B2, coenzyme Q10, cysteine as well as herbs, such as bilberry, turmeric (curcumin), grape seed or pine bark extracts, and ginkgo.